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About LED heating and cooling
Dec 28, 2016

With the recent years LED technology as a new generation of lighting technology has been widely concerned, LED power increases, heat issues are more and more attention. The researchers found that the long-term observation, this is because the LED decay or its life is directly related to its junction temperature, the heat is not good as high temperature, short life.

Unlike incandescent lamp and fluorescent lamp used in the past, their energy loss is great, but most of the energy is out through the infrared radiation source directly, less heat; and LED, as well as visible light energy consumption, other energy is converted into heat. Also due to electronic products gradually to high density, high integration development, Led products are no exception, so solve the problem of Led heat dissipation has become the main problem to improve Led performance, the development of Led industry.

LED cause of fever:

The reason for LED fever is that the added energy is not converted into light energy, but part of it is converted into heat energy. The luminous efficiency of LED is only 100lm/W, its electro-optic conversion efficiency is only about 20~30%. That is about 70% of the electricity is turned into heat.

Specifically, LED junction temperature is caused by two factors:

1 the internal quantum efficiency is not high, that is, when the electron and hole recombination, and can not produce photons, usually referred to as "current leakage", so that the recombination rate of the PN region carriers reduced 100%. Leakage current multiplied by the voltage is the power of this part, which is converted into heat, but this part does not account for the main component, because the internal photon efficiency is close to 90%.

2 internally generated photons can not be all ejected to the outside of the chip and finally converted to heat, which is part of the main, because this is called external quantum efficiency of only about 30%, most of them into heat.

As previously mentioned although incandescent light efficiency is very low, only about 15lm/W, but it will be almost all electrical energy into light energy and radiation out, because most of the infrared radiation is, so the light efficiency is very low, but with no heat problem.

LED cooling solution:

Solve Led heat dissipation, mainly from two aspects, before and after packaging, can be understood as Led chip cooling and Led lamp cooling. Led chip cooling mainly associated with the choice of substrate and circuit and process, because any LED will be made of lamps, so the heat generated by the LED chip is always through the shell of the lamp scattered into the air. If the heat is not good, because the LED chip heat capacity is very small, a little bit of heat accumulation will make the chip's junction temperature rapidly improve, if long time working in high temperature state, its life will quickly shorten. However, these heat to really guide the chip to reach the outside air, through many ways. Specifically, the LED chip produced by heat, from the metal radiating piece of it, after the first solder to aluminum substrate of PCB, and then through the heat conducting glue to aluminum radiator. So LED lamp cooling actually includes heat conduction and heat dissipation two parts.

However, the size of LED lamp cooling according to power and place, also have different choices. Mainly have the following heat dissipation method:

Aluminum fin heat dissipation: This is the most common way, using aluminum fins as a portion of the housing to increase the cooling area.

Conductive plastic shell: plastic shell filled with heat conducting material injection, increase the plastic shell heat conduction, heat dissipation capacity.

Air fluid mechanics: the use of shell shape, create convection air, this is the lowest cost way to strengthen the heat.

Fan: the lamp shell with long life and high efficiency fan strengthen heat dissipation, low cost, good effect. But to change the fan is more trouble, it is not suitable for outdoor, this design is rare.

The heat pipe using heat pipe technology, the heat from the LED chip to the housing guide fins. In large lamps, such as street lamps are common design.

Surface treatment: radiation lamp surface radiation treatment, simply smear Zhi Sheng Wei Hua radiation coating, heat can be taken away from the lamp surface with radiation.

Following the introduction of a new heat radiation cooling ZS-411 coating: coating, coating with high thermal conductivity and large radiating surface area, at the same time in a wide wavelength range (1-20 m) with high radiation rate, can significantly improve the comprehensive performance including conduction, convection and radiation heat.

The coating with high performance cooling solution, thermal infrared visible and near-infrared reflectance, higher emission rate and the stability of the special performance of the cooling solution is high, but also has excellent physical properties, chemical properties and good construction of a variety of complex, the working principle of the cooling solution by inorganic colloid particles (less than 100 nm) condensation combined by force. The heat conduction in the coating solution of carbon nanotubes with higher rate and emission of the material, can show the morphology of macro micro rough surface finish coating to the nano material component, can greatly increase the contact area of the cooling device with the outside world, significantly enhance the cooling effect. At the same time by adding a variety of spinel electronic transitions and the composite infrared radiator, which not only increase the impurity level, improve the infrared radiation coefficient, and maintain the thermal stability, the corresponding heat resistance.

Overall, the luminous efficiency of LED is still relatively low, resulting in increased junction temperature, reduced life. In order to reduce the junction temperature in order to improve the life must attach great importance to the issue of heat dissipation.

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